docs: def plot(ply):
      '''
      Plot vertices and triangles from a PlyData instance. Assumptions:
          `ply' has a 'vertex' element with 'x', 'y', and 'z'
              properties;
  
          `ply' has a 'face' element with an integral list property
              'vertex_indices', all of whose elements have length 3.
  
      '''
      vertex = ply['vertex']
  
      (x, y, z) = (vertex[t] for t in ('x', 'y', 'z'))
  
      mlab.points3d(x, y, z, color=(1, 1, 1), mode='point')
  
      if 'face' in ply:
          tri_idx = ply['face']['vertex_indices']
          idx_dtype = tri_idx[0].dtype
  
          triangles = numpy.fromiter(tri_idx, [('data', idx_dtype, (3,))],
                                     count=len(tri_idx))['data']
  
          mlab.triangular_mesh(x, y, z, triangles,
                               color=(1, 0, 0.4), opacity=0.5)